1. Technical Field
The present invention relates to a method for decoding a navigation message carried on a positioning signal, and the like.
2. Related Art
The GPS (Global Positioning System) is known as a satellite positioning system utilizing positioning signals and is broadly used in receiving devices installed inside mobile phones, car navigation systems and the like. In the GPS, a pseudo-range is found in order to calculate the position of a GPS receiving device. The pseudo-range is the distance from a GPS satellite to the GPS receiving device. In order to find the pseudo-range, the satellite orbit, accurate time information and the like are necessary. The information of the satellite orbit and the like is carried as a navigation message on a positioning signal at a bit rate of 50 bps (bits per second).
As a technique for decoding a navigation message from a positioning signal, for example, U.S. Patent Application Publication No. 2008/0069271 discloses a technique in which whether the bit value of a navigation message is changed or not is determined based on the plus or minus sign of an indicator value called an IQ inner product value (Dot value).
In circumstances where the speed of the GPS receiving device changes in short cycles, the related-art technique may not be able to decode a navigation message correctly. A typical situation thereof is where the user walks or runs, holding or wearing a GPS receiving device on his or her arm and swinging the arms. For example, when a mobile phone is considered as an electronic device equipped with a GPS receiving device, a situation where the user walks, swinging his or her hand holding the mobile phone, can be anticipated. Also, when a running watch is considered as an electronic device equipped with a GPS receiving device, a situation where the user runs, wearing the running watch on his or her arm, can be anticipated.
In such situations, the speed of the GPS receiving device changes in short cycles according to the user's swing of arms. As the speed of the GPS receiving device changes, the Doppler frequency in receiving a GPS satellite signal (positioning signal) changes. The Doppler frequency is one of primary factors that determine the receiving frequency of the GPS satellite signal. Meanwhile, after the GPS satellite signal is captured, tracking loop processing to predict and track changes in the receiving frequency starts. Therefore, if the Doppler frequency changes, the receiving frequency may be predicted in accordance with the change.
However, as in general signal tracking processing, the related-art tracking loop filter for GPS satellite signal is not designed to be able to follow frequency changes in short cycles. The tracking loop filter is designed in such a way that if the receiving frequency changes largely during tracking and the GPS satellite signal can no longer be received, it is assumed that the tracking is unlocked and therefore processing to recapture may be carried out. The fact that the GPS satellite signal can no longer be received can be determined based on the result of correlation calculation between C/A code and replica code.
Meanwhile, changes in the receiving frequency due to the user's swing of arms are cyclic and the cycle is short. Moreover, the relative size of the change frequency in relation to the receiving frequency is small. Therefore, the tracking seldom becomes unlocked. However, there is a discrepancy between the receiving frequency for tracking and the actual receiving frequency. This discrepancy is known to be a factor causing a decoding error in a navigation message.